The invention is in the field of propellers for marine outboard drive units and more specifically concerns a novel propeller having particular utility when enployed with drive units which employ underwater exhaust through the lower gear case.
In recent years outboard motors and marine stern drive units for pleasure boats have incorporated underwater engine exhaust systems which route the exhaust down through the drive shaft housing, aft through the lower gear case and out an annular passage through the propeller hub. These units are commonly called "through-the-hub exhaust" systems and are in common use around the world. One such system is presently manufactured by the Mercury Marine Division of Brunswick Corporation under the trademark of Jet-Prop.sup.TM, and a similar system is illustrated in U.S. Pat. No. 2,948,252 issued to Charles F. Alexander. The patent describes such an exhaust system for outboard motors.
Prior to the invention it was, and is, common practice to utilize through-the-hub-exhaust type (T-H-E) propellers, as described in the Alexander patent, with underwater exhaust systems. The hub exhaust passage of such propellers was specifically designed to keep the exhaust gases from causing the propeller to ventilate and the flaired hub creates a low pressure area immediately aft of the prop. This low pressure area reduces the exhaust back pressure on the engine and increases engine efficiency. The main objective of such systems has always been to reduce exhaust noise by underwater exhaust and at the same time minimize the effect of increased drag created by the increase in size of the lower gear case required to accommodate the exhaust passage.
Racing outboards are an exception to the above. While all stock outboards equipped with through-the-hub exhaust systems are sold with T-H-E propellers as standard equipment, such engines have been equipped with non-T-H-E propellers in special purpose configurations for stock class racing. When so equipped, the engines are preferably mounted high on the transom to reduce drag so that a good portion of the upper half of the propeller is out of the water when the boat is moving at high speed. Racing propellers used under these circumstances are designed to operate partially out of the water but are effective in driving light weight race boat hulls only and are not generally as suitable for use on medium to high speed pleasure boats for which the prop of the invention was developed.
One of the problems with drive units incorporating underwater exhaust systems using T-H-E propellers typical of the current art is that the combination is sensitive to variation in its height adjustment on the transom of the boat. More specifically such props are intolerant of the introduction of surface air under the "anti-cavitation" plate which occurs when the engine is mounted with the anti-cavitation plate higher than the boat bottom. While the optimum height adjustment depends primarily upon the hull design and operating speed, it was generally true that the anti-cavitation plate of the lower unit has to ride on the water so as to shield the prop from surface air, otherwise the prop will "break loose", ventilate and lose thrust. This most often occurs when accelerating onto a plane, in a turn, or when the unit is trimmed up (tilted about a horizontal axis) to achieve a more efficient or desired boat attitude.
A distinction should be made between a total "break loose" or "blow out" of the prop which is undesirable and normally requires a significant reduction in throttle to shed the air, exhaust, or cavitation bubbles, and a partial, ventilation which normally does not require a change in throttle to ultimately eliminate the condition when planing speed is reached. A primary advantage of the prop of the invention is that it can withstand a partial ventilation whether caused by underwater exhaust or other operational factors; so that its utility is not limited to T-H-E drive units.
Prior to the invention, non-T-H-E propellers were not considered suitable for general use on pleasure boats operated at moderate top speeds of between 30 and 50 mph, and, as mentioned above, drive units on non-racing hulls were always mounted with the anti-cavitation plate at or below the water level at planing speeds (at or below an extension of the boat bottom).
Through experimentation and test, the applicant has discovered that by proper use of blade rake and camber (hereinafter explained) a non-T-H-E propeller can be made to operate successfully on standard T-H-E outboards and stern drives driving medium to high speed pleasure boats with a number of advantages (hereinafter discussed) over use of the standard T-H-E propeller.
Another problem which had to be overcome before the non-T-H-E propeller could be used with standard T-H-E outboards or stern drives was that of weeds wrapping around the propeller drive shaft of hub forward of the blades. Weeds wrapped in this manner will block the exit of the exhaust passage from the lower gear case and choke the engine, so that it will die at speeds above idle. To solve this problem the invention includes weed cutting fingers radially off-set from the propeller shaft and extending axially between the blades and the exhaust outlet in the lower gear case housing. Also, the propeller blades are swept aft to more readily permit weeds to slide thereover and not be trapped forward of the blades where they will tend to wrap around the propeller drive shaft and hub. In some cases, particularly at a slow idle, weeds will wrap around the outside of the fingers; but as the fingers are radially off-set from the hub of the prop ample room is provided for the exhaust to escape. When the engine is accelerated, the higher rotational and forward velocities cause the loosely wrapped weeds to be cut up by the "finger" and thrown off.